1. Field of the Invention
The present invention relates to a method and system for improving the performance at low signal-to-noise ratios of receivers with Viterbi decoders. More particularly, the present invention provides a method and system for decoding in response to bit error rate.
2. The Background Art
Referring first to FIG. 1, a communication system having a digital signal transmission and receiving system is illustrated. A transmission portion of the digital signal transmission and receiving system includes an encoder 10 and a modulator 12 providing a modulated signal at a communication channel 14. Similarly, a receiving portion of the digital signal transmission and receiving system includes a demodulator 16 and a decoder 18.
The encoder 10 may be a convolutional encoder. Convolutional codes typically include redundant symbols to increase the signal to noise ratio. In this manner, the probability of errors introduced during encoding is minimized, increasing the probability of accurate transmission. A convolutional code may be described using a trellis diagram, which illustrates all possible code sequences.
Encoding schemes may provide a feedforward realization or feedback realization scheme. A feedforward realization is typically preferred for decoding purposes, since information is associated with a corresponding state, allowing information to be recovered directly. A feedback realization stores information in the encoded bits, making it more difficult to retrieve this information. As a result, encoding schemes typically provide a feedforward realization.
In a bandwidth-limited environment, a multilevel signaling scheme, such as phase-shift keying (PSK), may be used. Thus, the modulated signal includes an in-phase component and a quadrature component. When the modulated signal is received, after conversion from an analog to a digital signal by an analog-to-digital converter, each bit is demodulated into the in-phase and quadrature signal components by the demodulator 16 using sine and cosine functions.
The decoder 18 may comprise a Viterbi decoder, which may be used to decode these convolutional codes. The optimum method for decoding convolutional codes is the Viterbi algorithm, which provides an efficient method for searching a trellis diagram for the most likely transmitted code word. Viterbi decoders typically assume a feedforward realization encoding system. It would be desirable to provide a decoding system which would accomodate a feedback realization encoding scheme as well as a feedforward encoding scheme.
Referring now to FIG. 2, a typical bit error rate (BER) 20 versus signal-to-noise ratio (SNR) 22 curve 24 for coded and uncoded systems is presented. As shown, the bit error rate curve for an uncoded system 26 decreases as the SNR 22 increases. Similarly, the bit error rate for a coded system 28 decreases in a linear fashion as the SNR 22 increases. For all digital systems, there is a signal-to-noise ratio threshold 30 above which a coded system yields a lower bit error rate than an uncoded system. However, below this signal-to-noise ratio threshold 30, a coded system will yield a higher bit error rate than an uncoded system.
Standard decoding techniques decode received signal sequences without reference to the signal-to-noise ratio threshold or corresponding bit error rate threshold. When the power of the received signal sequences, or SNR, is lower than the signal-to-noise ratio threshold, the number of errors created by the channel noise exceed the capability of the convolutional code to correct these errors. As a result, the performance of a typical Viterbi decoder below a given signal-to-noise ratio threshold is worse than a communication system of an identical rate of an uncoded system. Thus, it would be highly beneficial if the bit error rate below this signal-to-noise ratio threshold could be systematically reduced in a coded system.